The Pool A maths are complicated and the permutations many, but hosts England go into the showdown with Australia on 3 October knowing defeat means an exit from the Rugby World Cup in the opening round for the first time. A week after their shock 28-25 defeat to Wales stopped their campaign in its tracks, the hosts must rouse themselves for yet another high pressure encounter at Twickenham.
"For us as a team and for us as individuals, you know it doesn't get much bigger than this," England flyhalf, Owen Farrell told reporters at a news conference on 1 October. "Well hopefully it does. Hopefully this does go on and gets bigger for us. But right now, this week, we have got to put everything we can into what we are doing. And like I said, he have done. And I think we have done it in the right way as well.
"We have not been too desperate to get things done too quickly, and frantic in the way we are doing it, or anything like that. We have been nice and calm and switched-on and gone about it in the right way."
England have indeed won four of their last five matches against Australia but Lancaster, and his opposite number Michael Cheika, are aware that World Cups are markedly different from June and November tours.
Lancaster has talent out wide and has brought more creativity into his backline with the recall from injury of Jonathan Joseph at outside centre and the relegation of rugby league convert Sam Burgess to the bench. It is, however, still on the forwards that England will pin their hopes of a victory that would keep alive their bid to win a second World Cup.
Cheika was given a wake-up call when he first took charge of a Wallabies side at Twickenham last year and England showed him that the weakness in the Australian pack was no mere perception but a potentially decisive fact. He has since overhauled his front five but acknowledges that the success or otherwise of his work can only be properly judged after the match.
Wales and Australia meet at Twickenham in their final Pool A game on 10 October, while England take on Uruguay in Manchester later the same day in what the entire host nation, and the tournament organisers, will be hoping is not a dead rubber.